In the image forming apparatus of the electro photographic system, an electrostatic latent image formed on an image carrying member is actualized with a toner by a developing device to be made into a toner image, the toner image is directly transferred on a recording medium or is transferred on the recording medium via an intermediate transcription member, and finally, this toner image is discharged to the outside together with the recording medium. Therefore, the toner is wasted as the number of printed images is increased. Detecting a toner concentration image transferred on the recording medium or the intermediate transcription member by a toner concentration sensor, if the toner concentration is lowered, the toner is replenished to the developing device from a toner hopper via a sub hopper.
When the toner is consumed, the toner state is changed as follows:
(1) A toner cartridge becomes vacant.
This state is judged by a replenishing amount counter of the toner cartridge.
(2) The sub hopper becomes vacant.
This state is judged by a remaining amount sensor of the sub hopper.
(3) The toner concentration on the recording medium or the intermediate transcription member is lowered.
This state is judged by the toner concentration sensor.
As a toner empty stop condition to prohibit printing due to toner empty, conventionally, there are two conditions, and each of them has a merit and a demerit, respectively.
[Condition 1] Empty Stop Due to Lowering of Toner Concentration (Sub Hopper is Vacant)
It is possible to extend interruption of printing. However, when the toner consumption is much, the toner concentration is lowered beyond the scope of assumption. In addition, upon release of empty, replenishing of the sub hopper does not catch up with the toner consumption caused by printing, and the toner concentration is further lowered, leading to a possibility that a trouble caused by abnormal lowering in the toner concentration is generated. In order to avoid this, it is necessary to delay restart of a job by replenishing the toner by compulsion.
[Condition 2] Empty Stop Due to Empty of Sub Hopper (Toner Concentration is Normal)
It is possible to avoid a risk of lowering of the toner concentration. However, even though there is a possibility of printing yet, a print job is interrupted.
In Patent Document 1, it is proposed to output a detection signal with respect to the remaining amount in a developer in a developer container of a cartridge and a caution signal with respect to the state such that the remaining amount in the developer within the cartridge is enough; the remaining amount in the developer has a quantity so as to affect an image quality; and the remaining amount in the developer damages the image quality or the like. In addition, in Patent Document 2, it is proposed to store a parameter for detecting the toner remaining amount that is different for each color to a nonvolatile memory of each cartridge, respectively, and detect the toner remaining amount that is different for each color. However, in any patent document, no condition relating to the toner empty stop is described.                Patent Document 1: JP Laid-Open Patent Publication No. 2002-268476        Patent Document 2: JP Laid-Open Patent Publication No. 2004-45447        